1. Field of the Invention
The present invention relates to a semiconductor integrated circuit having a protection element which is connected to an element or circuit to be protected (protected circuit) so as to remove input noise.
2. Description of the Related Art
Some semiconductor integrated circuits have a protection element adapted to perform electrostatic discharge (ESD) so as to protect the internal circuit from static electricity which finds its way into the circuit from an external terminal.
An ESD protection element is connected between two wirings connected to the internal circuit. One of such wirings is, for example, a supply line (hereinafter VDD line) of a source voltage VDD or a supply line (hereinafter VSS line) of a reference voltage VSS (e.g., GND voltage). The other of such wirings is, for example, a signal line. On the other hand, a power line discharge element (a type of ESD protection element) may be connected between the VDD and VSS lines to ensure positive ESD protection by discharging the charge of the VDD line to the VSS line.
A thyristor may be used as an ESD protection element (refer, for example, to Chih-Yao Huang etc., “Design Optimization of ESD protection and latchup prevention for a serial I/O IC,” Microelectronics Reliability 44 (2004) 213-221).
Some ESD protection elements have a single or a plurality of diodes connected between two wirings so as to face the same direction. This type of protection element will be hereinafter referred to as a “unidirectional diode protection element.”
If connected, for example, between the VDD and signal lines, a unidirectional diode protection element is connected in such an orientation as to turn ON when a voltage higher than the VDD line voltage is supplied to the signal line. On the other hand, if connected between the VSS and signal lines, a unidirectional diode protection element is connected in such an orientation as to turn ON when a voltage lower than the VSS line voltage is supplied to the signal line.
Some ESD protection elements use a GGMOS (Gate-Grounded MOSFET) provided between two wirings (refer, for example, to Japanese Patent No. 3327060). This GGMOS is formed simultaneously with the MOSFET making up the internal circuit.
A GGMOS protection element is a so-called diode-connected transistor having its gate and source shorted together. Therefore, this type of protection element has to be connected in proper orientation as with a diode. A GGMOS protection element is connected between two wirings in the same orientation as with the above unidirectional diode protection element.